usb: mtu3: add generic compatible string

The mtu3 driver is a generic driver for MediaTek usb3 DRD IP, add
a generic compatible to avoid confusion when support new SoCs but
use a compatible with specific SoC's name "mt8173".

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>